staging: pi433: Split subtraction across 2 lines

Split a subtraction across 2 lines in order to make these lines
no longer than 80 columns. Problem found with checkpatch.

Signed-off-by: Eisha Chen-yen-su <chenyensu0@gmail.com>
Acked-by: Julia Lawall <julia.lawall@lip6.fr>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>